Modeling, Control Design, and Analysis of a Startup Scheme for Modular Multilevel Converters

Featuring modularity and high efficiency, a modular multilevel converter (MMC) has become a promising topology in high-voltage direct-current transmission systems. However, its distributed capacitors lead to a more complicated startup process than that of a two-level converter. To fully understand t...

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Bibliographic Details
Published in:IEEE transactions on industrial electronics (1982) 2015-11, Vol.62 (11), p.7009-7024
Main Authors: Xiaojie Shi, Bo Liu, Zhiqiang Wang, Yalong Li, Tolbert, Leon M., Fei Wang
Format: Article
Language:English
Subjects:
Capacitors
Control systems
Converters
Electric potential
highvoltage direct-current (HVDC) transmission systems
Integrated circuits
Inverters
Metal matrix composites
Modular
Modular multilevel converter (MMC)
Modulation
Process control
Rectifiers
startup
Strategy
Systems stability
Voltage control
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Summary:Featuring modularity and high efficiency, a modular multilevel converter (MMC) has become a promising topology in high-voltage direct-current transmission systems. However, its distributed capacitors lead to a more complicated startup process than that of a two-level converter. To fully understand this issue, the charging loops of an MMC rectifier and an MMC inverter during an uncontrolled precharge period are analyzed in this paper, with special focus on the necessity of additional capacitor charging schemes. Moreover, a small-signal model of a capacitor charging loop is first derived according to the internal dynamics of the MMC inverter. Based on this model, a novel startup strategy incorporating an averaging capacitor voltage loop and a feedforward control is proposed, which is capable of an enhanced dynamic response and system stability without sacrificing voltage control precision. The design considerations of the control strategy are also given in detail. Simulation results from a back-to-back MMC system supplying passive loads and experimental results from a scaled-down MMC prototype are provided to support the theoretical analysis and the proposed control scheme.
ISSN:0278-0046
1557-9948
DOI:10.1109/TIE.2015.2436354